Corrosion of metals can be a significant problem where metal surfaces are exposed to water, air, inorganic metal salts, and the like. One of the most severe sources of corrosion is a seawater environment, that is, seawater itself, seawater mist or spray, or the salty air which can exist near bodies of seawater. A seawater environment can attack and corrode ferrous and non-ferrous metals.
A variety of equipment is subject to corrosion by seawater, including piping, dry-docks, steel cables, superstructures of an equipment on offshore drilling and production rigs, desalination plants, seawater mineral extracting plants, surfaces of ships and barges, and tanks subjected to seawater environment.
Such equipment is quite large in surface area, but is not subject to severe frictional and abrasive force. It requires an inexpensive, yet effective, coating to protect its ferrous metal from the seawater environment. Soft skin coatings which do not contain expensive pigments are useful for this purpose. Although such coatings are not hard, they do provide a "skin" which protects the metal surface from the seawater environment.
Many soft skin coatings are known, such as those disclosed in U.S. Pat. Nos. 3,925,087, 4,495,225, 4,675,215, and 4,729,791, which provide corrosion protection from aqueous environments for metal surfaces. All of these patents describe soft skin coatings which contain specified combinations of (a) an overbased alkaline earth organic aliphatic or mixed one ring aliphatic-aromatic sulfonate salt, (b) a drying oil, (c) a metal drier, (d) a paraffinic oil, and possibly (e) a solvent. These coatings disclosed provide corrosion resistance for metals which are exposed to aqueous environments, particularly saltwater environments. The thickness of the coatings is usually no greater than 10 mils, but is typically, 1 to 3 mils.
A well known and commercially successful ballast coating within the scope of U.S. Pat. No. 3,925,087 is MAGNAKOTE.RTM. ballast tank coating. This commercial coating is highly effective in inhibiting the corrosion of ballast tanks and other large surface area equipment made of ferrous metals which is exposed to a seawater environment.
In general, a thicker coating will last longer and provide better corrosion resistance than a thinner coating of an equally effective substance, but is more expensive and takes longer to apply. Additionally, when a thicker layer is applied, the coating requires an inconveniently long period of time to cure. In addition, a thicker layer tends to flow, and a coating thickness of greater than about 3 or 4 mils generally cannot be maintained. Further, while these coatings are considered the best available from an economic level for inhibiting the corrosion of ferrous metal surfaces, they are typically useful for only a period of up to two years before a new coat is required. Consequently, longer-lasting corrosion protection is highly desirable.
Accordingly, a need exists for a corrosion inhibiting soft skin coating which will maintain a coating thickness of preferably 3 mils or less, which will cure rapidly, resist sagging, and exhibit exceptionally effective and long-lasting corrosion inhibition.